1. Field of the Invention
This invention relates to electrophotography and in particular to coloration resistant photoconductive insulating compositions and elements and to processes using such compositions and elements.
2. Description of the Prior Art
The process of xerography, as disclosed by Carlson in U.S. Pat. No. 2,297,691, employs an electrophotographic element comprising a support material bearing a coating of an insulating material whose electrical resistance varies with the amount of incident electromagnetic radiation it receives, such as during an image-wise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge, generally in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of this surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or discharge pattern as desired. Deposited marking material can then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor or the like, or transferred to a second element to which it can similarly be fixed. Likewise, the electrostatic charge pattern can be transferred to a second element and developed there.
Various photoconductive insulating materials have been employed in the manufacture of electrophotographic elements. For example, vapors of selenium and vapors of selenium alloys deposited on a suitable support and particles of photoconductive zinc oxide held in a resinous, film-forming binder have found wide application in present-day document copying processes.
Since the introduction of electrophotography, a great many organic compounds have also been screened for their photoconductive properties. As a result, a very large number of organic compounds have been known to possess some degree of photoconductivity. Many organic compounds have revealed a useful level of photoconduction and have been incorporated into photoconductive compositions. Among these organic photoconductors are certain of the triphenylamines as described in U.S. Pat. No. 3,180,730 issued Apr. 27, 1965, and the polyarylalkane compounds such as those described in U.S. Pat. No. 3,274,000 issued Sept. 20, 1966; U.S. Pat. No. 3,542,547 issued Nov. 24, 1974; and in U.S. Pat. No. 3,615,402 issued Oct. 26, 1971.
Electrophotographic elements on which marking material is deposited and permanently affixed are often called direct recording or direct imaging materials. It is desirable that such materials exhibit no color or very low coloration in non-image background areas. As an example, it has long been an object to minimize background stain in electrophotographic papers, such as those intended for office copying or for making copies from microfilm such as on reader/printer equipment. In inorganic photoconductive materials, photoconductive metal oxides are often white in appearance. However, it has been difficult to prepare coloration-resistant (non-stain-producing) organic photoconductive materials, which can provide a considerable advantage over the inorganic photoconductive materials that are often weighty and unpleasant to handle. Such difficulty occurs for reasons such as: (1) interactions between the binder and the photoconductor that impart color to the element beyond that of the constituents, due to absorption of the reaction product in the visible region of the spectrum and (2) inherently poor light stability possessed by many of the most efficient organic photoconductors which tend to form coloration upon prolonged exposure to conventional room light. Additionally, background coloration is much more apparent and easily discerned by the naked eye when compositions are coated on white reflective supports than when coated on transparent supports.
Various dyes useful in sensitizing photoconductors have been described. U.S. Pat. Nos. 3,554,745 and 3,577,235 describe benzopyrylium sensitizers as useful in forming electrophotographic materials of lower background color when used in conjunction with, respectively, organic amine or amino substituted photoconductors and organometallic photoconductors. No suggestion of the relationship of reflective support materials to the problem of background coloration is made in these patents, nor is there any reference to nitrogen-free photoconductors that might reduce still further the amount of background color.
U.S. Pat. No. 3,246,983 describes certain ethylene derivative photoconductors. Such photoconductors are hydrogen substituted, as distinguished from the ethylene derivative photoconductors described herein which are tetraaryl substituted. Further, there is no indication in U.S. Pat. No. 3,246,983 of the ability to form coloration resistant photoconductive insulating compositions as described herein.
The problem of background color formation can be illustrated by preparing, using conventional techniques, a homogeneous electrographic paper in which the photoconductive layer includes Vitel.sup.R 101 polyester binder and 4,4'-diethylamino-2,2'-dimethyltriphenylmethane as a photoconductor. Neither of these compounds, taken separately, exhibits any appreciable absorption in the visible region of the spectrum. However, when they are mixed to form a photoconductive composition, a yellowish charge-transfer complex is produced. After several days exposure to normal office illumination, this electrographic paper changes to a green color.
A patent application of C. J. Fox, PHOTOCONDUCTIVE INSULATING COMPOSITIONS INCLUDING POLYARYL HYDROCARBON PHOTOCONDUCTORS, filed concurrently herewith U.S. Ser. No. 595,955, describes the combination of certain photoconductors derived from ethylene, butadiene and hexatriene with pyrylium salts and Lewis acids as sensitizers. Although the photoconductors of the Fox application include certain of the nitrogen-free photoconductors described in detail herein, there is no suggestion in the Fox application that such photoconductors, or the additional photoconductors of the present photoconductive compositions, could be combined with benzopyrylium sensitizers to produce coloration resistant photoconductive insulating compositions. The subject matter of this application is considered to provide desirable properties and results not suggested in the Fox application.